The present invention relates generally to micro-electromechanical systems (MEMS), and in particular to MEMS that include torsionally actuated devices employing comb-drive actuators and methods of fabricating and using such actuators and devices.
Micro-electromechanical systems (MEMS) technology has been used increasingly in the development of many micro devices such as optical switches. MEMS technology can utilize lithographic mass fabrication processes used by the semiconductor industry in manufacturing integrated circuits (ICs). MEMS based optical switches typically consist of arrays of tiny mirrors and are found in two-dimensional (2D) and three-dimensional (3D) varieties. Two-dimensional MEMS based mirrors only can tilt in two positions, up or down, whereas 3D MEMS based mirrors can tilt in any direction, i.e., up, down or to the sides.
To move the tiny mirrors, comb-drive actuators can be used. Single-layer and staggered vertical comb-drive actuators have been used to produce torsional motion of structures through the application of electrostatic forces. As shown in FIG. 1, comb-drive actuators include two components: a mobile comb-finger structure 309 and a stationary comb-finger structure 312. Each comb structure has a plurality of fingers interdigiated with the other comb structure. By applying a voltage potential across the structures (which are isolated from one another), an electrostatic field is developed between the structures. In one position, the stationary comb-like structure 312 and the mobile comb-like structure 309 partly overlap as shown in FIG. 1. When the potential is applied, the electrostatic field causes the mobile structure 309 to move so as to maximize the overlap between the two comb-finger structures 309, 312.
A single-layer vertical comb-drive actuator is disclosed in U.S. Pat. No. 5,696,848 issued to Lee et al. A staggered vertical comb-drive actuator capable of larger vertical motion compared to single-layer vertical comb-drive actuator is disclosed in international patent publications WO 01/73934 A2, WO 01/73935 A2, WO 01/73936 A2, WO 01/73937 A2, WO 01/74707 A2 and WO 01/76055 A2 to Behin et al.
In the references listed above, the staggered vertical comb-drive actuator provides a larger vertical/rotational motion compared to the single-layer comb-drive actuator. In the single-layer vertical comb-drive actuator, the maximum vertical motion is about 1.5 micrometers resulting in a very small angle of rotation. The single-layer comb-drive actuator consists of a single conducting layer where the mobile comb-like structure is attracted toward the stationary comb-like structure once a bias voltage is applied between them. The staggered comb-drive actuator consists of two conducting layers separated by an insulating layer or air gap. The top and bottom layers can be biased so that the top layer of the mobile comb-like structure is attracted toward the bottom layer of the stationary comb-like structure thus applying a larger torque on the mobile side. The voltage profile across the staggered actuator depth is uniform within each layer and its value is equal to the biasing voltage. However, the staggered vertical comb-drive actuator lacks the capability to create a non-uniform voltage profile across the actuator depth and to dynamically change such voltage profile across the comb-drive depth during operation. Providing such capability permits more versatile designs with larger vertical/rotational motion. In addition, it is desirable to have a single-layer comb-drive actuator that can provide a larger angle of rotation in comparison to single-layer comb-drive actuators.
Referring to FIG. 1, the vertical comb-drive actuator 313 applies the force directly on a rotating element 302 suspended over a cavity 305 by flexures 307. The comb-drive actuator 313 is located off of the rotational axis of the element 302 by a distance R. In this example, the vertical height d (i.e. depth) of the staggered comb-drive actuator 313 depends on both the angle of rotation xe2x88x9d, the actuator length L 316, and the distance R 315 between the mobile comb-like structure 309 and the flexures 307. Specifically, the comb height must be larger than (L+R).tan(xe2x88x9d) assuming that there is enough torque available to achieve the desired angle of rotation xe2x88x9d. Increasing R results in a larger torque applied on the mobile comb-like structure 309. Thus, relatively large and massive comb-drive actuators are needed for large angles of rotation.
This leads to some potential drawbacks. First, a large comb-drive actuator can lead to lower resonance frequency. Further, as the height of the comb drive increases the width of comb fingers as well as the gap between adjacent mobile and stationary comb fingers have to be increased accordingly due to the limited verticality (typically 90xc2x10.5 degrees) of the deep reactive ion etch (RIE) process, usually used to create such structures. The increased gap leads to a smaller force exerted by each stationary comb finger 311 on the corresponding mobile comb finger 308 and results in smaller capacitance values between mobile 308 and stationary 311 comb fingers. The smaller capacitance leads to position sensors of lower accuracy and/or more complicated measurement techniques of capacitance.
In addition, this larger gap and wider comb fingers lead to a smaller number of comb fingers that can be formed in a given space resulting in a further reduction in the overall torque acting on the mobile comb-like structure 309. Thus, a smaller angle of rotation is achieved or higher voltage is needed to maintain the desired angle of rotation. These conflicting performance and size demands are inherent to the vertical comb-drive actuator 313.
Therefore, there is a need for new types of rotating comb-drive actuators and torsional micro-mirror systems that improve on known actuators and systems in terms of smaller size, higher resonant frequency, larger angle of rotation, lower actuation voltage, more precise position sensing, and simpler fabrication methods.
It is an advantage of the present invention to provide an improved comb-drive actuator and system employing the same. It is a further advantage of the invention to provide a smaller size rotating vertical comb-drive actuator with higher resonance frequency, larger angle of rotation, lower actuation voltage and/or more precise position sensors with less complex measurement schemes.
In accordance with an embodiment of the invention, a pn-based vertical comb-drive actuator includes a p-type semiconducting layer on top of a n-type semiconducting layer or vice versa. The stationary and mobile comb-like structures have the same pn-structure. The pn-structure in the stationary comb-like structure is reverse biased. Whereas in the mobile comb-like structure, either the n-type or p-type layer is grounded in order to establish an electric field between the opposite semiconducting types of the stationary and mobile comb-like structures. This causes the stationary comb-like structure to apply a torque on the mobile comb-like structure resulting in a vertical and/or rotational motion.
In accordance with another embodiment of the invention, an improved actuator system is achieved by applying the torque directly on the flexures suspending a torsional element over a cavity, rather than applying such torque on the torsional element itself at an off-axis location.
Other embodiments of the invention provide uni-axial and multi-axial systems employing comb-drive actuators, as well as fabrication methods and operational procedures for the actuators, position sensors, and systems.
Other embodiments, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features, embodiments and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.